CHAPTER 37WEATHEROBSERVATIONSBASICSOFWEATHEROBSERVATIONS3700.Introductionthin metal cell which is compressed byatmospheric pres-sure, slight changes in air pressure cause the cell to expandWeather forecasts are generallybased upon informa-or contract,while a systemoflevers magnifies and convertstion acquired by observations made at a large number ofthismotion to a reading on a gage or recorderEarlymercurial barometers were calibrated to indicatestations.Ashore,these stations are located so astoprovideadequatecoverageofthe areaofinterest.Mostobservationsthe height, usually in inches or millimeters, of the columnatseaaremadebymariners,wherevertheyhappentobeofmercuryneededtobalancethecolumnofairabovetheSincethe number of observations at sea is small comparedpoint ofmeasurement.While units of inches and millime-tothe numberashore,marine observations areof great im-ters are still widely used, many modern barometers areportance.Data recorded by designated vessels are sent bycalibratedtoindicatethecentimeter-gram-second unitofradio to weather centers ashore, where they are plotted,pressure,themillibar, which is equal to 1,000 dynes peralong with other observations,to providedata for drawingsquare centimeter.A dyne is the force required to acceleratesynoptic charts, which are used to makeforecasts.Com-a mass of one gram at the rate of one centimeterper secondplete weather information gathered at sea by cooperatingper second.A reading in any of the three units of measure-vessels is mailedto the appropriate meteorological servicesmentcanbeconvertedtotheequivalentreadingineitheroffor use in the preparation of weather atlases and in marinethe other units by means oftables, or the conversionfactorsclimatological studies.given in the appendix. However, the pressure readingA special effort should be made to provideroutine syn-should always be reported in millibars.optic reportswhen transiting areas wherefew ships areavailable to report weather observations.This effort is par-3702.TheBarometerticularlyimportant inthetropics,where a vessel's synopticThe mercurial barometerwas invented by Evangelis-weather report may be one of the first indications of a de-veloping tropical cyclone. Even with satellite imagery,ta Torricelli in 1643.In its simplest form it consists of aactual reports are needed toconfirm suspicious patterns andglasstubealittlemorethan30inchesinlengthandofuniprovide actual temperature,pressure, and other measure-form internal diameter.With one end closed, thetube isments.Forecasts can be no better than thedatareceived.filled withmercury,and inverted intoacupofmercury.Themercury in the tubefalls until the column is just supported3701.AtmosphericPressurebythepressureof the atmosphereontheopen cup,leavinga vacuum at the upper end of the tube.The height of the col-The sea ofair surrounding the earth exerts a pressure ofumn indicates atmospheric pressure,greater pressuresabout 14.7 pounds per square inch on the surface of thesupportinghigher columns ofmercury.earth.This atmosphericpressure,sometimes calledbaro-The mercurial barometer is subject torapid variationsmetric pressure,varies from place to place, and at thein height,called pumping,dueto pitch androll of thevessameplace itvariesover time.sel and temporary changes in atmospheric pressure in theAtmosphericpressure isoneofthemostbasicelementsvicinity of the barometer.Because of this,plus the care re-of ameteorological observation.When thepressure ateachquired in the reading the instrument, its bulkiness, and itsstation isplotted on a synopticchart, linesof equal atmo-vulnerability to physical damage, the mercurial barometerhas been replaced at sea by the aneroid barometer.spheric pressure, called isobars,indicate the areas of highand lowpressure.Theseareuseful inmakingweatherpre3703.TheAneroidBarometerdictions,becausecertaintypesofweatherarecharacteristicof eachtypeof area,and the windpatterns overlarge areascanbededucedfrom the isobars.The aneroid barometermeasures theforce exerted byAtmospheric pressure is measured withabarometer.atmosphericpressureonapartly evacuated,thin-metal ele-A mercurial barometermeasurespressurebybalancingmentcalled a sylphon cell (aneroid capsule).A small springthe weight of a column of air against that of a column ofis used, either internally or externally,to partly counteractthe tendency oftheatmospheric pressureto crushthecellmercury.The aneroid barometer has a partly evacuated,521
521 CHAPTER 37 WEATHER OBSERVATIONS BASICS OF WEATHER OBSERVATIONS 3700. Introduction Weather forecasts are generally based upon information acquired by observations made at a large number of stations. Ashore, these stations are located so as to provide adequate coverage of the area of interest. Most observations at sea are made by mariners, wherever they happen to be. Since the number of observations at sea is small compared to the number ashore, marine observations are of great importance. Data recorded by designated vessels are sent by radio to weather centers ashore, where they are plotted, along with other observations, to provide data for drawing synoptic charts, which are used to make forecasts. Complete weather information gathered at sea by cooperating vessels is mailed to the appropriate meteorological services for use in the preparation of weather atlases and in marine climatological studies. A special effort should be made to provide routine synoptic reports when transiting areas where few ships are available to report weather observations. This effort is particularly important in the tropics, where a vessel’s synoptic weather report may be one of the first indications of a developing tropical cyclone. Even with satellite imagery, actual reports are needed to confirm suspicious patterns and provide actual temperature, pressure, and other measurements. Forecasts can be no better than the data received. 3701. Atmospheric Pressure The sea of air surrounding the earth exerts a pressure of about 14.7 pounds per square inch on the surface of the earth. This atmospheric pressure, sometimes called barometric pressure, varies from place to place, and at the same place it varies over time. Atmospheric pressure is one of the most basic elements of a meteorological observation. When the pressure at each station is plotted on a synoptic chart, lines of equal atmospheric pressure, called isobars, indicate the areas of high and low pressure. These are useful in making weather predictions, because certain types of weather are characteristic of each type of area, and the wind patterns over large areas can be deduced from the isobars. Atmospheric pressure is measured with a barometer. A mercurial barometer measures pressure by balancing the weight of a column of air against that of a column of mercury. The aneroid barometer has a partly evacuated, thin metal cell which is compressed by atmospheric pressure; slight changes in air pressure cause the cell to expand or contract, while a system of levers magnifies and converts this motion to a reading on a gage or recorder. Early mercurial barometers were calibrated to indicate the height, usually in inches or millimeters, of the column of mercury needed to balance the column of air above the point of measurement. While units of inches and millimeters are still widely used, many modern barometers are calibrated to indicate the centimeter-gram-second unit of pressure, the millibar, which is equal to 1,000 dynes per square centimeter. A dyne is the force required to accelerate a mass of one gram at the rate of one centimeter per second per second. A reading in any of the three units of measurement can be converted to the equivalent reading in either of the other units by means of tables, or the conversion factors given in the appendix. However, the pressure reading should always be reported in millibars. 3702. The Barometer The mercurial barometer was invented by Evangelista Torricelli in 1643. In its simplest form it consists of a glass tube a little more than 30 inches in length and of uniform internal diameter. With one end closed, the tube is filled with mercury, and inverted into a cup of mercury. The mercury in the tube falls until the column is just supported by the pressure of the atmosphere on the open cup, leaving a vacuum at the upper end of the tube. The height of the column indicates atmospheric pressure, greater pressures supporting higher columns of mercury. The mercurial barometer is subject to rapid variations in height, called pumping, due to pitch and roll of the vessel and temporary changes in atmospheric pressure in the vicinity of the barometer. Because of this, plus the care required in the reading the instrument, its bulkiness, and its vulnerability to physical damage, the mercurial barometer has been replaced at sea by the aneroid barometer. 3703. The Aneroid Barometer The aneroid barometer measures the force exerted by atmospheric pressure on a partly evacuated, thin-metal element called a sylphon cell (aneroid capsule). A small spring is used, either internally or externally, to partly counteract the tendency of the atmospheric pressure to crush the cell
522WEATHEROBSERVATIONSoWEATER710AROMETEELANEORLPMANUM2552Figure3703.AnaneroidbarometerAtmospheric pressureis indicated directlybya scalechartto indicatethepressureatanytimeandapointerconnectedtothecellbyacombinationofle-Thebarograph is usually mounted on a shelfor desk invers.The linkage provides considerable magnification ofaroom opentotheatmosphere,in alocationwhich mini-the slight motion of the cell, to permit readings to highermizes the effect of the ship's vibration. Shock-absorbingprecision than could be obtained without it.material such as sponge rubber may be placed under the in-Ananeroidbarometershouldbemountedpermanent-strumenttominimizevibration.ly.Prior to installation,the barometer should be carefullyThepen shouldbe checked and the inkwell filled eachset.U.S.shipsoftheVoluntaryObservingShip(VOS)pro-time the chart is changed.gram are set to sea level pressure. Other vessels may be setA marine microbarograph is a precision barographto stationpressureand corrected for heightasnecessary.Anusing greater magnification and an expanded chart.It is de-adjustmentscrewisprovidedforthispurpose.Theerrorofsigned to maintain its precision through the conditionsthe instrument is determined by comparison witha mercu-rial barometerora standard precision aneroid barometer.Ifencounteredaboardship.Twosylphoncellsareused,onemounted over the other in tandem.Minor fluctuations duea qualified meteorologist is not available to make this ad-justment,adjust by first removing only one-half theto shocks or vibrations are eliminated by damping.Sinceapparent error.Thetap the casegentlyto assist the linkageoil-filled dashpots are used forthis purpose, the instrumentto adjust itself, and repeat the adjustment. If the remainingshould never be inverted. The dashpots of the mi-error is not more than half amillibar (0.015 inch), no at-crobarograph should be kept filled with dashpot oil totempt should be made to remove it by further adjustment.within three-eighths inch of the topInstead,a correction should be applied to thereadings.TheShip motions are compensated by damping and springaccuracyof thiscorrectionshouldbecheckedfromtimetoloading which make it possible for the microbarograph totime.betilted upto22°withoutvaryingmorethan0.3millibarsfrom truereading.Microbarographs havebeen almost en-3704.The Barographtirelyreplacedbystandard barographsBoth instruments require checkingfrom time to time toThe barograph is a recording barometer. In principleinsure correct indication ofpressure.The position of theit is the same as a nonrecordinganeroid barometer exceptpen is adjusted by a small knob provided for this purpose.that the pointer carries a pen at its outer end, and the scaleTheadjustment should bemade in stages, eliminatinghalfis replaced by a slowly rotating cylinder around which achart is wrapped.A clock mechanism inside the cylinder ro-the apparent error,tapping the caseto insure linkage adjust-tatesthecylindersothatacontinuouslineistracedonthementto the new setting,andthenrepeating theprocess
522 WEATHER OBSERVATIONS Atmospheric pressure is indicated directly by a scale and a pointer connected to the cell by a combination of levers. The linkage provides considerable magnification of the slight motion of the cell, to permit readings to higher precision than could be obtained without it. An aneroid barometer should be mounted permanently. Prior to installation, the barometer should be carefully set. U.S. ships of the Voluntary Observing Ship (VOS) program are set to sea level pressure. Other vessels may be set to station pressure and corrected for height as necessary. An adjustment screw is provided for this purpose. The error of the instrument is determined by comparison with a mercurial barometer or a standard precision aneroid barometer. If a qualified meteorologist is not available to make this adjustment, adjust by first removing only one-half the apparent error. The tap the case gently to assist the linkage to adjust itself, and repeat the adjustment. If the remaining error is not more than half a millibar (0.015 inch), no attempt should be made to remove it by further adjustment. Instead, a correction should be applied to the readings. The accuracy of this correction should be checked from time to time. 3704. The Barograph The barograph is a recording barometer. In principle it is the same as a nonrecording aneroid barometer except that the pointer carries a pen at its outer end, and the scale is replaced by a slowly rotating cylinder around which a chart is wrapped. A clock mechanism inside the cylinder rotates the cylinder so that a continuous line is traced on the chart to indicate the pressure at any time. The barograph is usually mounted on a shelf or desk in a room open to the atmosphere, in a location which minimizes the effect of the ship’s vibration. Shock-absorbing material such as sponge rubber may be placed under the instrument to minimize vibration. The pen should be checked and the inkwell filled each time the chart is changed. A marine microbarograph is a precision barograph using greater magnification and an expanded chart. It is designed to maintain its precision through the conditions encountered aboard ship. Two sylphon cells are used, one mounted over the other in tandem. Minor fluctuations due to shocks or vibrations are eliminated by damping. Since oil-filled dashpots are used for this purpose, the instrument should never be inverted. The dashpots of the microbarograph should be kept filled with dashpot oil to within three-eighths inch of the top. Ship motions are compensated by damping and spring loading which make it possible for the microbarograph to be tilted up to 22° without varying more than 0.3 millibars from true reading. Microbarographs have been almost entirely replaced by standard barographs. Both instruments require checking from time to time to insure correct indication of pressure. The position of the pen is adjusted by a small knob provided for this purpose. The adjustment should be made in stages, eliminating half the apparent error, tapping the case to insure linkage adjustment to the new setting, and then repeating the process. Figure 3703. An aneroid barometer
523WEATHEROBSERVATIONS3705.Adjusting Barometer Readings3706.TemperatureTemperature is ameasure ofheat energy,measured inAtmospheric pressure as indicated bya barometer orbarograph may be subject to several errors.degrees.Several different temperature scales are in use.Instrument error:Inaccuracy due to imperfection orOn the Fahrenheit (F) scale pure water freezes at 320incorrectadjustmentcanbedeterminedbycomparison withand boils at 2120a standard precision instrument.The National Weather Ser-OntheCelsius(C)scalecommonlyusedwiththemet-viceprovides a comparison service.In major U.S.ports aric system, the freezing pointof pure water is 0°and thePort Meteorological Officercarriesaportableprecisionan-boiling point is 10ooThis scale,has beenknownbyvariouseroidbarometerforbarometercomparisonsonboardshipsnames in different countries. In the United States it wasfor-whichparticipateintheVoluntaryObservingShip(VOS)merly called the centigrade scale. The Ninth Generalprogram of theNational Weather Service.The portableba-Conference of Weights and Measures, held in France inrometer is compared with stationbarometersbefore and1948,adopted thenameCelsiustobeconsistentwiththeaftera shipvisit.Ifa barometeristaken to aNationanaming of othertemperature scales after their inventorsWeather Serviceshore station,the comparison can bemadeand toavoid the use of different names indifferent coun-there.The correct sea-level pressure can also be obtained bytries.OntheoriginalCelsiusscale.invented in1742byatelephone.TheshipboardbarometershouldbecorrectedforSwedish astronomer named Anders Celsius,numberingheight,as explained below,beforecomparison withthiswas thereverse of the modern scale, oorepresenting thevalue. If there is reason to believe that the barometer is inboiling point ofwater, and 100oits freezing point.error,it should becompared witha standard,and ifan errorAbsolutezerois consideredtobethelowestpossibleis found,the barometer should be adjusted to the correcttemperature, at which there is no molecular motion and areading, or a correction applied to all readings.bodyhas no heat.For somepurposes,it is convenienttoex-Height error: The atmospheric pressure reading at thepress temperature by a scale at which oo is absolute zero.height of the barometer is called the station pressure andThis is called absolute temperature.If Fahrenheit degreesis subject to a height correction in order to make it a sea lev-are used, it maybecalledRankine (R)temperature;and ifel pressure readingIsobars adequately reflect windCelsius, Kelvin (K) temperature. The Kelvin scale is moreconditions and geographic distribution of pressure onlywidelyused than theRankine.Absolutezero is-459.69°Fwhentheyaredrawnforpressureatconstantheight(ortheor-273.16°C.varyingheightatwhichaconstantpressureexists).Onsyn-Temperatureofone scalecanbeeasilyconverted toan-opticcharts itiscustomaryto showtheequivalentpressureother because of the linear mathematical relationshipat sea level, called sea level pressure.This isfound by ap-between them. Notethat the sequence of calculation isplying a correction to station pressure. The correctionslightly different, algebraic rules must befollowed.depends upon the height of the barometer and the averageC = F-32temperature of the air between this height and the surfaceC =(F-32),or1.8Theoutsideairtemperaturetaken aboard shipis sufficientlyaccurateforthispurpose.This is an importantcorrection0F = 1.8C+32F=C + 32. orwhich should beappliedtoall readingsof anytypebarom-eter.SeeTable31forthis correction.K =C+273.16Gravity error: Mercurial barometers are calibrated forstandard sea-level gravity at latitude 45°32'40".If the gravityR = F + 459.69differsfrom this amount, an error is introduced.The correc-tion to be applied to readings at various latitudes is given inA temperature of-40° is the same by either the CelsiusTable32.This correctiondoesnotapplytoreadingsofanan-eroid barometer or microbarograph.Gravity also changesorFahrenheitscale.Similarformulascanbemadeforconwith height above sea level, but the effect isnegligiblefor theversionofother temperaturescalereadings.TheConversionTableforThermometer Scales (Table29)gives the equiva-firstfew hundredfeet,and so is notneededfor readings takenlentvaluesofFahrenheit,Celsius,andKelvintemperatures.aboard ship.See Table 32 for this correction.Temperature error: Barometers are calibrated at aThe intensity or degree of heat (temperature)should notstandard temperatureof32F.The liquidof amercurialba-be confused with the amount of heat.If the temperature of airrometerexpandsasthetemperatureofthemercuryrises.andorsomeothersubstanceistobeincreased(thesubstancemadecontracts as it decreases.The correction to adjust the readinghotter)by a given number of degrees, the amount of heat thatoftheinstrumenttothetruevalueisgiven inTable33.Thismust be added is dependent upon theamountof the substancetobeheated.Also,equal amounts ofdifferentsubstances recorrectionis appliedtoreadingsofmercurial barometersonly.Modernaneroidbarometersarecompensatedfortem-quiretheaddition ofunequalamounts ofheattoeffectanequalperature changes by the use of different metals havingincreaseintemperaturebecauseoftheirdifferenceof specificheat.Units used for measurement of amount of heat are theunequalcoefficients of linearexpansion
WEATHER OBSERVATIONS 523 3705. Adjusting Barometer Readings Atmospheric pressure as indicated by a barometer or barograph may be subject to several errors. Instrument error: Inaccuracy due to imperfection or incorrect adjustment can be determined by comparison with a standard precision instrument. The National Weather Service provides a comparison service. In major U. S. ports a Port Meteorological Officer carries a portable precision aneroid barometer for barometer comparisons on board ships which participate in the Voluntary Observing Ship (VOS) program of the National Weather Service. The portable barometer is compared with station barometers before and after a ship visit. If a barometer is taken to a National Weather Service shore station, the comparison can be made there. The correct sea-level pressure can also be obtained by telephone. The shipboard barometer should be corrected for height, as explained below, before comparison with this value. If there is reason to believe that the barometer is in error, it should be compared with a standard, and if an error is found, the barometer should be adjusted to the correct reading, or a correction applied to all readings. Height error: The atmospheric pressure reading at the height of the barometer is called the station pressure and is subject to a height correction in order to make it a sea level pressure reading. Isobars adequately reflect wind conditions and geographic distribution of pressure only when they are drawn for pressure at constant height (or the varying height at which a constant pressure exists). On synoptic charts it is customary to show the equivalent pressure at sea level, called sea level pressure. This is found by applying a correction to station pressure. The correction depends upon the height of the barometer and the average temperature of the air between this height and the surface. The outside air temperature taken aboard ship is sufficiently accurate for this purpose. This is an important correction which should be applied to all readings of any type barometer. See Table 31 for this correction. Gravity error: Mercurial barometers are calibrated for standard sea-level gravity at latitude 45°32’40". If the gravity differs from this amount, an error is introduced. The correction to be applied to readings at various latitudes is given in Table 32. This correction does not apply to readings of an aneroid barometer or microbarograph. Gravity also changes with height above sea level, but the effect is negligible for the first few hundred feet, and so is not needed for readings taken aboard ship. See Table 32 for this correction. Temperature error: Barometers are calibrated at a standard temperature of 32°F. The liquid of a mercurial barometer expands as the temperature of the mercury rises, and contracts as it decreases. The correction to adjust the reading of the instrument to the true value is given in Table 33. This correction is applied to readings of mercurial barometers only. Modern aneroid barometers are compensated for temperature changes by the use of different metals having unequal coefficients of linear expansion. 3706. Temperature Temperature is a measure of heat energy, measured in degrees. Several different temperature scales are in use. On the Fahrenheit (F) scale pure water freezes at 32° and boils at 212°. On the Celsius (C) scale commonly used with the metric system, the freezing point of pure water is 0° and the boiling point is 100°. This scale, has been known by various names in different countries. In the United States it was formerly called the centigrade scale. The Ninth General Conference of Weights and Measures, held in France in 1948, adopted the name Celsius to be consistent with the naming of other temperature scales after their inventors, and to avoid the use of different names in different countries. On the original Celsius scale, invented in 1742 by a Swedish astronomer named Anders Celsius, numbering was the reverse of the modern scale, 0° representing the boiling point of water, and 100° its freezing point. Absolute zero is considered to be the lowest possible temperature, at which there is no molecular motion and a body has no heat. For some purposes, it is convenient to express temperature by a scale at which 0° is absolute zero. This is called absolute temperature. If Fahrenheit degrees are used, it may be called Rankine (R) temperature; and if Celsius, Kelvin (K) temperature. The Kelvin scale is more widely used than the Rankine. Absolute zero is –459.69°F or –273.16°C. Temperature of one scale can be easily converted to another because of the linear mathematical relationship between them. Note that the sequence of calculation is slightly different; algebraic rules must be followed. A temperature of –40° is the same by either the Celsius or Fahrenheit scale. Similar formulas can be made for conversion of other temperature scale readings. The Conversion Table for Thermometer Scales (Table 29) gives the equivalent values of Fahrenheit, Celsius, and Kelvin temperatures. The intensity or degree of heat (temperature) should not be confused with the amount of heat. If the temperature of air or some other substance is to be increased (the substance made hotter) by a given number of degrees, the amount of heat that must be added is dependent upon the amount of the substance to be heated. Also, equal amounts of different substances require the addition of unequal amounts of heat to effect an equal increase in temperature because of their difference of specific heat. Units used for measurement of amount of heat are the C 5 9 = C -( ) F 32 – , or F 32 – 1.8 = - F 9 5 = F 1.8 -C 32 or + , = C + 32 K C 273.16 = + R F 459.69 = +
524WEATHEROBSERVATIONSBritish thermal unit (BTU),the amount of heat needed toThe same process causes moisture to form on the out-raisethetemperature of 1poundof water1Fahrenheit,andside of a container of cold liquid, the liquid cooling the airthe calorie,the amount ofheatneeded to raise thetemperatureintheimmediatevicinityofthecontaineruntilitreachestheof 1 gram of water 1° Celsius.dewpoint.When moisture is deposited on man-madeob-jects, it is usually called sweat. It occurs whenever the3707.Temperature Measurementtemperature of a surface is lower than the dew point of airin contact with it. It is of particular concern to themarinerbecause of its effect upon his instruments,and possibleTemperature is measured with a thermometer.Mostdamagetohis ship oritscargo.Lensesofoptical instruthermometers are based upon the principle that materials ex-ments may sweat, usuallywithsuch small dropletsthatthepand with an increase of temperature, and contract assurface has a“"frosted"appearance. When this occurs, thetemperaturedecreases.In its most usualformathermometerinstrument is said to“fog"or“fog up,"and is useless untilconsistsofabulbfilledwithmercuryand connectedtoatubethemoisture is removed.Damage is often caused bycorro-of very small cross-sectional area.Themercury onlypartlysion or direct water damage when pipes sweat and drip,orfills the tube. In the remainder is a vacuum. Air is driven outwhen the inside of the shell plates of a vessel sweat.Cargoby boiling the mercury,and the top ofthe tube is then sealed.may sweat if it is cooler than the dew point oftheair.As themercury expands orcontracts withchanging temper-Clouds andfog form from condensation of wateronature, the length of the mercury column in the tube changes.minute particles of dust, salt, and other material in the air.Sea surface temperature observations are used in theEachparticleforms anucleus aroundwhichadropletofwa-forecastingoffog and furnish important information aboutterforms.If air is completely free from solid particles onthedevelopmentand movement oftropical cyclones.Com-which water vapor may condense, the extra moisture re-mercial fishermen are interested in the sea surfacemains inthevapor state,andtheairis saidtobetemperature as an aid in locating certain species of fish.supersaturated.There are severalmethods of determining seawatertemper-Relative humidity and dew point are measured witha hyature.Theseincludeengineroomintakereadings,condensergrometer.The most common type, called a psychrometer,intake readings, thermistor probes attached to the hull, andreadings from bucketsrecovered from overtheside.Al-consistsoftwothermometersmountedtogetheronasinglestripofmaterial.Oneofthethermometersismountedalittlethoughthecondenserintakemethodisnotatruemeasureoflowerthantheother,and has itsbulb covered withmuslin.surface water temperature, the error is generally smallWhen themuslin covering is thoroughly moistened and theIf the surfacetemperature isdesired,a sample shouldthermometerwell ventilated,evaporationcoolsthebulbofthebeobtained by bucket,preferablya canvas bucket,fromathermometer, causing it to indicate a lower reading than theforward position well clear of any discharge lines.The sam-other.A sling psychrometer is ventilated by whirling theple should be taken immediately to a place where it isthermometers.Thedifferencebetweenthedrv-bulbandwet-sheltered from wind and sun. The water should then bebulb temperatures is used to enter psychrometric tables (Ta-stirred with the thermometer,keeping thebulb submerged,ble35andTable36)tofindtherelativehumidityanddewuntil a constant reading is obtained.point. If the wet-bulb temperature is abovefreezing,reason-Aconsiderablevariationinseasurfacetemperaturecanably accurateresultscan be obtained by a psychrometerbe experienced in a relatively shortdistance of travel.Thisconsistingof dry-and wet-bulb thermometersmounted soisespeciallytruewhen crossingmajor ocean currents suchthat air can circulate freely around them without special ven-astheGulfStreamand theKuroshioCurrent.Significanttilation.Thistypeofinstallation is common aboard shipvariations also occur where large quantities of freshwaterare discharged from rivers.A clevernavigator will noteExample:The dry-bulbtemperature is 65°F,and thethesechangesasinindicationofwhentoallowforsetandwet-bulb temperature is 61°F.drift in dead reckoning.Required: (l)Relative humidity, (2)dew point.Solution: The difference between readings is 4o. En-3708.HumidityteringTable35withthisvalue,andadry-bulbtemperatureof65°,therelativehumidityisfoundtobe80percent.FromHumidity is a measure ofthe atmosphere's water vaporTable36thedewpoint is58°content. Relative humidity is the ratio, stated as a percent-Answers:()Relativehumidity80percent,(2)dewage,ofthepressureofwatervaporpresent intheatmospherepoint58°tothesaturationvaporpressureatthesametemperatureAs air temperature decreases, the relative humidity in-creases.Atsomepoint,saturationtakesplace,andanyfurtherAlso in use aboard many ships is the electric psy-cooling results in condensation of some of themoisture.Thechrometer.This is a hand held, battery operated instrumenttemperature at which this occurs is called the dew point, andwith two mercury thermometers for obtaining dry-and wet-bulbtemperature readings.Itconsists of a plastichousingthemoisture deposited upon objects is called dewif it forms intheliquid state, orfrost ifitforms in thefrozen state.that holds the thermometers, batteries, motor, and fan
524 WEATHER OBSERVATIONS British thermal unit (BTU), the amount of heat needed to raise the temperature of 1 pound of water 1° Fahrenheit; and the calorie, the amount of heat needed to raise the temperature of 1 gram of water 1° Celsius. 3707. Temperature Measurement Temperature is measured with a thermometer. Most thermometers are based upon the principle that materials expand with an increase of temperature, and contract as temperature decreases. In its most usual form a thermometer consists of a bulb filled with mercury and connected to a tube of very small cross-sectional area. The mercury only partly fills the tube. In the remainder is a vacuum. Air is driven out by boiling the mercury, and the top of the tube is then sealed. As the mercury expands or contracts with changing temperature, the length of the mercury column in the tube changes. Sea surface temperature observations are used in the forecasting of fog and furnish important information about the development and movement of tropical cyclones. Commercial fishermen are interested in the sea surface temperature as an aid in locating certain species of fish. There are several methods of determining seawater temperature. These include engine room intake readings, condenser intake readings, thermistor probes attached to the hull, and readings from buckets recovered from over the side. Although the condenser intake method is not a true measure of surface water temperature, the error is generally small. If the surface temperature is desired, a sample should be obtained by bucket, preferably a canvas bucket, from a forward position well clear of any discharge lines. The sample should be taken immediately to a place where it is sheltered from wind and sun. The water should then be stirred with the thermometer, keeping the bulb submerged, until a constant reading is obtained. A considerable variation in sea surface temperature can be experienced in a relatively short distance of travel. This is especially true when crossing major ocean currents such as the Gulf Stream and the Kuroshio Current. Significant variations also occur where large quantities of freshwater are discharged from rivers. A clever navigator will note these changes as in indication of when to allow for set and drift in dead reckoning. 3708. Humidity Humidity is a measure of the atmosphere’s water vapor content. Relative humidity is the ratio, stated as a percentage, of the pressure of water vapor present in the atmosphere to the saturation vapor pressure at the same temperature. As air temperature decreases, the relative humidity increases. At some point, saturation takes place, and any further cooling results in condensation of some of the moisture. The temperature at which this occurs is called the dew point, and the moisture deposited upon objects is called dew if it forms in the liquid state, or frost if it forms in the frozen state. The same process causes moisture to form on the outside of a container of cold liquid, the liquid cooling the air in the immediate vicinity of the container until it reaches the dew point. When moisture is deposited on man-made objects, it is usually called sweat. It occurs whenever the temperature of a surface is lower than the dew point of air in contact with it. It is of particular concern to the mariner because of its effect upon his instruments, and possible damage to his ship or its cargo. Lenses of optical instruments may sweat, usually with such small droplets that the surface has a “frosted” appearance. When this occurs, the instrument is said to “fog” or “fog up,” and is useless until the moisture is removed. Damage is often caused by corrosion or direct water damage when pipes sweat and drip, or when the inside of the shell plates of a vessel sweat. Cargo may sweat if it is cooler than the dew point of the air. Clouds and fog form from condensation of water on minute particles of dust, salt, and other material in the air. Each particle forms a nucleus around which a droplet of water forms. If air is completely free from solid particles on which water vapor may condense, the extra moisture remains in the vapor state, and the air is said to be supersaturated. Relative humidity and dew point are measured with a hygrometer. The most common type, called a psychrometer, consists of two thermometers mounted together on a single strip of material. One of the thermometers is mounted a little lower than the other, and has its bulb covered with muslin. When the muslin covering is thoroughly moistened and the thermometer well ventilated, evaporation cools the bulb of the thermometer, causing it to indicate a lower reading than the other. A sling psychrometer is ventilated by whirling the thermometers. The difference between the dry-bulb and wetbulb temperatures is used to enter psychrometric tables (Table 35 and Table 36) to find the relative humidity and dew point. If the wet-bulb temperature is above freezing, reasonably accurate results can be obtained by a psychrometer consisting of dry- and wet-bulb thermometers mounted so that air can circulate freely around them without special ventilation. This type of installation is common aboard ship. Example: The dry-bulb temperature is 65°F, and the wet-bulb temperature is 61°F. Required: (1) Relative humidity, (2) dew point. Solution: The difference between readings is 4°. Entering Table 35 with this value, and a dry-bulb temperature of 65°, the relative humidity is found to be 80 percent. From Table 36 the dew point is 58°. Answers: (1) Relative humidity 80 percent, (2) dew point 58°. Also in use aboard many ships is the electric psychrometer. This is a hand held, battery operated instrument with two mercury thermometers for obtaining dry- and wetbulb temperature readings. It consists of a plastic housing that holds the thermometers, batteries, motor, and fan
525WEATHEROBSERVATIONS3709.Wind Measurementhas an apparent speed equal to the speed ofthe vessel. Thus,iftheactual ortrue wind is zero and the speed ofthevessel isWindmeasurement consists of determination ofthedi-10knots, theapparent wind is fromdead ahead at 10knotsrection and speed of the wind.Direction is measured byaIfthetruewind isfrom dead ahead at15knots,andthe speedwind vane, and speed by an anemometer.of the vessel is 10 knots, the apparent wind is 15 + 10 = 25Several types of wind speed and direction sensors areknots from dead ahead.If the vessel reverses course,theap-available, using vanes to indicate wind direction and rotat-parent wind is 15-10=5 knots, from dead astern.ingcups orpropellers for speed sensing.Manyships haveThe apparentwind isthevector sum of thetrue windreliable wind instruments installed,and inexpensive windand the reciprocal of the vessel's course and speed vector.instruments areavailableforeven thesmallestyacht.If noSincewind vanes and anemometers measure apparentanemometeris available,windspeedcanbeestimatedbyitswind, theusual problem aboarda vessel equipped with aneffect upon the seaand nearbyobjects.Thedirection can beanemometeristoconvertapparentwindtotruewind.Therecomputed accurately,even on a fastmovingvessel, byma-are several ways of doing this. Perhaps the simplest is byneuveringboardorTable30thegraphical solution illustrated inthefollowing example:3710.TrueAndApparentWindExampleI:A ship isproceedingon course240°ataspeed of 18knots.Theapparentwind is from 040°relativeAn observeraboard a vessel proceedingthrough still airat30knots.experiences an apparent wind which is from dead ahead andRequired:The direction and speed of the true wind.MANEUVERINGBOARDSCALESH130AopereetWiod-Figure3710.Finding true wind byManeuveringBoard
WEATHER OBSERVATIONS 525 3709. Wind Measurement Wind measurement consists of determination of the direction and speed of the wind. Direction is measured by a wind vane, and speed by an anemometer. Several types of wind speed and direction sensors are available, using vanes to indicate wind direction and rotating cups or propellers for speed sensing. Many ships have reliable wind instruments installed, and inexpensive wind instruments are available for even the smallest yacht. If no anemometer is available, wind speed can be estimated by its effect upon the sea and nearby objects. The direction can be computed accurately, even on a fast moving vessel, by maneuvering board or Table 30. 3710. True And Apparent Wind An observer aboard a vessel proceeding through still air experiences an apparent wind which is from dead ahead and has an apparent speed equal to the speed of the vessel. Thus, if the actual or true wind is zero and the speed of the vessel is 10 knots, the apparent wind is from dead ahead at 10 knots. If the true wind is from dead ahead at 15 knots, and the speed of the vessel is 10 knots, the apparent wind is 15 + 10 = 25 knots from dead ahead. If the vessel reverses course, the apparent wind is 15 – 10 = 5 knots, from dead astern. The apparent wind is the vector sum of the true wind and the reciprocal of the vessel’s course and speed vector. Since wind vanes and anemometers measure apparent wind, the usual problem aboard a vessel equipped with an anemometer is to convert apparent wind to true wind. There are several ways of doing this. Perhaps the simplest is by the graphical solution illustrated in the following example: Example 1: A ship is proceeding on course 240° at a speed of 18 knots. The apparent wind is from 040° relative at 30 knots. Required: The direction and speed of the true wind. Figure 3710. Finding true wind by Maneuvering Board